download: Allow keeping .git dirs with NODELETE=git

Keeping the git repositories is useful while development, e.g. to avoid
git cloning repositories over and over again while debugging download
scripts. Setting the NODELETE environment variable keeps the blobs and
the git repositories. Allow a slightly finer-tuned version of this where
we can keep only the git-related files by setting the variable to "git".

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>